The field of the invention is gyromagnetic resonance spectroscopy, and particularly, nuclear magnetic resonance (NMR) techniques for measuring the properties of materials.
Gyromagnetic resonance spectroscopy is conducted to study nuclei that have magnetic moments and electrons which are in a paramagnetic state. The former is referred to in the art as nuclear magnetic resonance (NMR), and the latter is referred to as paramagnetic resonance (EPR) or electron spin resonance (ESR). There are other forms of gyromagnetic spectroscopy that are practiced less frequently, but are also included in the field of this invention.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmour frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus).
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z) the individual magnetic moments of the paramagnetic nuclei in the tissue attempt to align with this field, but precess about it in random order at their characteristic Larmour frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field but the randomly oriented components in the perpendicular plane (x-y plane) cancel one another. If, however, the substance, or tissue, is irradiated with a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmour frequency, the net aligned moment, M.sub.z, can be rotated into the x-y plane to produce a net transverse magnetic moment M.sub.1 which is rotating in the x-y plane at the Lamour frequency. The degree to which the rotation of M.sub.z into an M.sub.1 component is achieved, and hence, the magnitude and the direction of the net magnetic moment (M=M.sub.0 +M.sub.1) depends primarily on the length of time of the applied excitation field B.sub.1.
The practical value of this gyromagnetic phenomena resides in the radio signal which is emitted after the excitation signal B.sub.1 is terminated. When the excitation signal is removed, an oscillating sine wave is induced in a receiving coil by the rotating field produced by the transverse magnetic moment M.sub.1. The frequency of this signal is the Larmour frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of M.sub.1. The amplitude A of the emission signal (in simple systems) decays in an exponential fashion with time, t: EQU A=A.sub.o e.sup.-t/T.sbsp.2.
The decay constant 1/T.sub.2 is a characteristic of the process and it provides valuable information about the substance under study. The time constant T.sub.2 is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant, and it measures the rate at which the aligned precession of the nuclei dephase after removal of the excitation signal B.sub.1.
Other factors contribute to the amplitude of the free induction decay (FID) signal which is defined by the T.sub.2 spin-spin relaxation process. One of these is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value M.sub.0 along the axis of magnetic polarization (Z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances, and its independent measurement is the subject of many gyromagnetic procedures.
The measurements described above are called "pulsed NMR measurements." They are divided into a period of excitation and a period of emission. As will be discussed in more detail below, this measurement cycle may be repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A variety of preparative excitation techniques are known which involve the application of one or more excitation pulses of varying duration. Such preparative excitation techniques are employed to "sensitize" the subsequently observed free induction decay signal (FID) to a particular phenomena. Some of these excitation techniques are disclosed in U.S. Pat. Nos. 4,339,716; 4,345,207; 4,021,726; 4,115,730 and 3,474,329.
Although NMR meansurements are useful in many scientific and engineering fields, their potential use in the field of medicine is enormous. NMR measurements provide a contrast mechanism which is quite different from x-rays, and this enables differences between soft tissues to be observed with NMR which are completely indiscernible with x-rays. In addition, physiological differences can be observed with NMR measurements, whereas x-rays are limited primarily to anatomical studies.
For most medical applications utilizing NMR, an imaging technique must be employed to obtain gyromagnetic information at specific locations in the subject. The foremost NMR imaging technique is referred to as "zeugmatography" and was first proposed by P. C. Lauterbur in a publication "Image Formation by Induced Local Interactions: Examples Employing Nuclear Magnetic Resonance", Nature, Vol. 242, Mar. 16, 1973, pp. 190-191. Zeugmatography employs one or more additional magnetic fields which have the same direction as the polarizing field B.sub.0, but which have a nonzero gradient. By varying the strength (G) of these gradients, the net strength of the polarizing field B.sub.0 =B.sub.z +G.sub.x X+G.sub.y Y+G.sub.z Z at any location can be varied. As a result, if the frequency reponse of the receiver is narrowed to respond to a single frequency, W.sub.o, then gyromagnetic phenomena, will be observed only at a location where the net polarizing field B.sub.0 is of the proper strength to satisfy the Larmour equation; W.sub.0 =.gamma.B.sub.0 : where W.sub.0 is the Larmour frequency at that location.
By "linking" the resulting free induction signal FID with the strengths of the gradients (G=G.sub.x, G.sub.y, G.sub.z) at the moment the signal is generated, the NMR signal is "tagged", or "sensitized", with position information. Such position sensitizing of the NMR signal enables an NMR image to be produced by a series of measurements.
The series of free induction decay signals produced during a scan of the subject are digitized and processed by a computer to extract their various frequency components for display on a screen. The most prevalent method involves the application of a discrete Fourier transform to the digitized NMR signals. Such transform may be in one or several variables as discussed in "The Fourier Transform and Its Applications", by R. N. Bracewall, published in 1978 by McGraw-Hill. Computer programs for performing such discrete Fourier transforms are well known, as discussed in "Fourier Analysis of Time Series: An Introduction", by P. Bloomfield, published in 1976 by Wiley. Two files of digital data are produced by the Fourier transformation of the time domain NMR signals. One file represents the "real" component and the second file represents the "imaginary" component. As discussed in U.S. Pat. No. 4,070,611 it can be demonstrated that the imaginary file is not required to reproduce an accurate image of the NMR phenomena of interest, and it is common practice to ignore this data.
The use of NMR to measure the flow of fluids in vessels is well known. A paper "The NMR Blood Flowmeter-Theory and History" by J. H. Battocletti et al, published in Medical Physics, Vol. 8, No. 4, July/August, 1981, describes the theory and history of this effort. The techniques heretofore employed to measure flow require special NMR apparatus with coils arranged to magnetize a sample of the fluid "upstream" of the coils which are employed to sense the FID signal. The physical distance between this "tagging" coil and the sensing coil is known, and the level of the FID signal provides velocity information in the direction of fluid flow. In an article "NMR Rheotomography: Feasibility and Clinical Potential", by J. P. Grant et al and published in Medical Physics, Vol. 9, No. 2, March/April 1982, imaging techniques are employed to provide a flow intensity distribution in a tube. Such techniques are limited to measuring flow in a known direction, and have been limited in practice to the measurement of flow in inanimate objects or to the measurement of blood flow in the arms and legs of animals.